MacBook Air ha 2 core del processore indipendenti.
Quando si utilizza Hyper Threading, ogni core può agire come due. In questo caso vedrai quattro core computazionali.
La pagina di EveryMac per MacBook Air spiega perché stai vedendo quattro in alcune situazioni e due nell'applicazione System Information:
This model is powered by a 22 nm, 64-bit Intel Mobile Core i7 "Ivy Bridge" (I7-3667U) processor which includes two independent processor "cores" on a single silicon chip. Each core has a dedicated 256k level 2 cache, shares 4 MB of level 3 cache, and has an integrated memory controller (dual channel).
This system also supports "Turbo Boost 2.0" -- which "automatically increases the speed of the active cores" to improve performance when needed (up to 3.2 GHz for this model) -- and "Hyper Threading" -- which allows the system to recognize four total "cores" or "threads" (two real and two virtual).